This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Growth failure is a significant manifestation of acquired prolonged hypothyroidism, resulting in permanent loss in height and only seventy percent catch-up growth is achieved with thyroxine replacement. Patients treated with thyroxine at a mean chronologic age of 11 years have adult heights approximately two standard deviations below the mean-final heights and are lower than the mid-parental and predicted adult heights. The concomitant administration of growth hormone to restore growth rate, in addition to a Gonadrotrophin releasing (GNRHa) to arrest epiphysical maturation, offers the possibility of improving final height prognosis. This randomized study will evaluate the long-term effect of concomitant administration of growth hormone and GNRHa in addition to thyroxine vs. standard therapy of thyroxine alone on the final height of adolescents with prolonged juvenile hypothyroidism. In addition, a cohort of patients with short duration of hypothyroidism will also be followed to delineate the effects of short vs. prolonged hypothyroidism on growth.